To find out how much wall area could be covered with 15 gallons of paint, we first need to determine the area that can be covered by 1 gallon of paint.
According to the problem, 5 gallons of paint cover 750 square feet. We can calculate the coverage per gallon:
\[ \text{Coverage per gallon} = \frac{750 \text{ sq ft}}{5 \text{ gallons}} = 150 \text{ sq ft per gallon} \]
Now, we can find out how much area can be covered by 15 gallons of paint:
\[ \text{Total coverage with 15 gallons} = 15 \text{ gallons} \times 150 \text{ sq ft per gallon} = 2250 \text{ sq ft} \]
Therefore, 15 gallons of paint can cover 2250 square feet of wall area.